In a mobile communication system, a service area is covered by a plurality of Base Stations (BTS). An existing antenna of a base station has a single column of antennas, where there is only a longitudinal feed network but no horizontal feed network with a directivity diagram of approximately a regular hexagon. The coverage area of a base station adopting such antennas is like a cell of a beehive, and thus an existing mobile communication system is called as a “Cellular System”. A mobile communication system may have numerous cells, and investment for that is very high, usually as high as tens or even hundreds of billions. Apparently, if each base station covers a larger area, i.e. the area of a cell is larger, the total number of base stations will be reduced, and thus the investment cost will be lowered. However, the capacity of each base station is limited, so its capacity density will be decreased with an enlarged coverage area. New base stations may need to be established with the number of users increasing, which is called network capacity expansion. With new base stations established, the area covered by an old cell should be diminished so as to make space for the newly-established base stations. This is called network splitting.
An ideal networking approach is to have larger coverage areas for individual base stations when initially establishing a network so as to reduce initial investment (because the number of registered users increases regularly from zero), and thus diminish capital to be invested and risk of investment, and to expand the capacity step by step with the number of users increasing, i.e. to split the network step by step. However, capacity expansion by means of network splitting requires additional necessary equipment, results in extra costs, and requires complex reconfiguration of local networks. Consequently, the network capacity expansion is neither “smooth” nor cost effective. Accordingly, the low initial investment conflicts with the high capacity expansion cost.
An existing wireless cellular system is networked basically in a three-sector form. This approach is advantageous in that 1) optimal match is obtained between the directivity diagram of antennas and a coverage area; and 2) cells are relatively normative, and directivities of antennas between base stations are staggered, thus minimizing interference between cells of the network. FIG. 1 is a schematic diagram of three-sector coverage, in which the circle indicates a location of a base station, and the arrows indicate directivities of an antenna of the base station. In such a structure, directivities of antennas are staggered. This is a favorable networking approach, and thus has been widely applied.
In a mobile communication system, as the number of users increases to some extent, new base stations are required for capacity expansion. However, for a wireless communication system, especially a 3G wireless communication system in compliance with WCDMA and CDMA2000 standards, capacity expansion by means of establishing new base stations requires addition of necessary equipment, and also causes extra cost. For instance, a new base station needs an additional site which gives rise to site costs. Moreover, because the coverage in a 3G system relates to capacity, both network planning and engineering are complex. Therefore, adding a new base station requires network planning that should consider both the new base station to be established and existing base stations around it, which results in new problems. Accordingly, for an addition of a new base station, there may be a “jump” with respect to costs, technologies and engineering, and consequently, such capacity expansion is not smooth.
For instance, to divide a coverage area of a base station in FIG. 1 into three areas, the topology after capacity expansion should be as illustrated in FIG. 2. When a new base station is added, coverage areas will be changed, and existing antennas shall be replaced with antennas of new specifications directivities and tilt angles of which must be reconfigured depending on antennas of the newly established base station. Furthermore, because the coverage areas are not normative, it may be difficult to obtain a good coverage for some dead zones regardless of adjusting. Therefore, to maintain a three-sector network structure, both locations and coverage areas of original base stations shall be changed, which will pose a great impact upon the original network.
A existing “one-off” network planning principle is commonly employed to avoid the above unsmooth capacity expansion, i.e. a network is planned and established considering capacity demand in future several years. This planning approach ensures that the network capacity will be sufficient in the coming several years, and therefore no capacity expansion will be needed. However, this means that a sufficient number of base stations shall be deployed at the beginning of constructing a wireless network. An advantage of this approach is that it satisfies a requirement for the number base stations in the coming years, while its disadvantage is that the initial investment will be large, which may bring an investment challenge to an operator. Moreover, the total number of base stations fails to be reduced, and the total cost of base station sites can not be lowered.
In this regard, the existing-adopted “one-off” networking approach is a solution that has to be employed to avoid the problem of capacity expansion with a premise that the problem of smooth capacity has not been solved, and thus has the following drawbacks:
1. Large initial investment: the “One-off” planning is to ensure that the network capacity will be sufficient in the coming years without any capacity expansion, and this means that more cellular networks shall be established at one time, which results in large initial investment and increased financial cost and risk;
2. High costs for base station sites: each base station involves some fixed costs such as cost of site building, accumulators, and tenancy. The more the base station sites, the higher the costs. Because the “one-off” planning requires more base stations, the costs of base station sites are higher.
3. Difficulty in planing: the “one-off” planning requires more base stations, but sometimes there is no appropriate site available for a base station at a place intended for the base station, which renders the network planning difficulty.
4. The existing solution just evades the problem of capacity expansion temporarily. The capacity will be saturated after years, so that the problem of capacity expansion with high costs will be inevitable.